The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Lateral, vertical and torsional loads transmitted via the tyre to the rim portion of a vehicle wheel result in bending and torsional stresses through the spokes. These stresses must be resolved efficiently at the hub join to provide a mechanically efficient structure that provides stiffness and strength, ideally without adding considerable mass to the wheel.
Existing composite vehicle wheel designs known to the Applicant do not meet these fundamental requirements.
The Applicant is aware of an earlier attempt to produce a composite wheel. However, the Applicant considers the design of that wheel is deficient to the extent that it incorporates metal for connection between the spoke and hub portions of the wheel rather than using a composite material for the connection.
One of the Applicant's previous composite wheel designs included continuous carbon fibre tows running straight across the wheel from spoke to spoke.
Another of their designs used an alternative tow based preform structure, with the wheel configured such that the lug holes were provided between spokes, rather than being more centrally located within the hub. This configuration maintained stiffness from the spoke to the hub because the depth of the spoke was maintained through the hub region of the wheel. In comparison to earlier attempts by others, this arrangement was more structurally efficient, and avoided the need to incorporate joining structures made from a different material (such as metal) using either adhesive or a fastener.
That said, the Applicant's designs referred to above required the tows to be arranged so as to avoid the central hole in the wheel. A central hole is provided for positioning on the vehicle hub. In some circumstances this resulted in the tows being machined to produce the holes, and in all cases the alignment could not be completely straight from one side of the rim to the other. The tows were often severed at the boltholes that were subsequently machined into the wheel hub. All of these issues compromised the efficiency and integrity of the final wheel structure.
Moreover, the tow solution was also difficult to lay into the mould with accuracy and consistency leading to unacceptable process variation.
Destructive testing also identified issues between the tow regions and other regions of the wheel structure that the Applicant considered were due to a mismatch in stiffness. Under high loads cracks were found to form at the interface between very stiff and more compliant regions in the structure.
It would therefore be desirable to provide a composite wheel of improved strength and stiffness in the region where the spokes meet the hub region. Moreover, it would be desirable to do in the absence of adding considerable mass to the wheel.